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Optical radiometer feasibility study for determining composition of Mars atmosphere from shock layer radiation during entry - instrumentation
Ablation product absorption and line transitions effects on shock layer radiative transport around blunt body
This paper reports experimental characterization of shock layer radiation from H 2 /He mixtures with a small amount of methane (CH 4 ) addition in the Electric Arc Shock Tube at NASA Ames Research Center. The work follows on a previous test series for entry probes to Saturn and Uranus, which quantified the expected radiance in the absence of methane. However, the Saturn stratosphere is estimated to have approximately 0.4% of CH 4 . The current test series utilized mixtures of 0.886/0.110/0.004 H 2 /He/CH 4 (by mole.) Freestream pressures included 0.1 and 0.5 Torr and velocities from 15-26 km/s. Data compares favorably to similar conditions collected in the Oxford T6 facility. Additional radiation features attributable to CH and C 2 Swan bands are observed, albeit at low intensities. Comparisons to data collected in previous EAST tests suggest that the CH 4 does not significantly alter reaction kinetics below 26 km/s.
Radiative transfer upstream absorption effect on inviscid stagnation region shock layer radiation
Radiative heating by the shock layer gases surrounding an entry probe is an important heating mechanism for many planetary entries. The magnitude of radiative heating for planetary atmospheres may be characterized with flight similarity in a shock tube capable of obtaining flight relevant velocities and atmospheric density/composition. Over the last decade, studies performed in the Electric Arc Shock Tube (EAST) at NASA Ames have characterized the radiative heating magnitudes for most relevant planetary destinations: Mars, Venus, Titan, Saturn, Uranus. Predictions of heating for a given entry trajectory are simulated by a combination of hypersonic flowfield modeling and the radiation code, NEQAIR, and then compared to the EAST test data. This paper will discuss the measurements, summarize the current simulation capability and provide indications for destination specific radiative heating uncertainties.
Finite difference techniques used in numerical solutions for inviscid radiating shock layers around spheres at hyperbolic speed in air
A computational technique of coupling radiative transfer to fluid motion is developed for axisymmetric blunt body shock layer flows in a thermochemical nonequilibrium environment. The coupled formulation of radiation and flowfield leads to a governing set of integro-differential equations. This equation set is solved using a modified Gauss-Seidel line relaxation techniques which incorporates the inversion of full block matrix associated with radiative transfer using a block iteration method. The thermodynamic state of the gas is described by three temperatures: translational, rotational, and vibrational-electronic. Radiation phenomenon is assumed to be governed by the vibrational-electronic temperature. The radiative properties are described by a spectrally detailed model. The computations are presented for two cases, including the Fire II flight experiment. It is shown that the method converges and the calculated spectra qualitatively agree with the experimental data for the two test cases. The calculated total radiative flux underestimates the measured values owing to the low vibrational-electronic temperature predicted in the flowfield calculation.
Measurements of the vibrational temperature in a shock layer produced by a cylindrical blunt body in an arc jet nitrogen flow are made for both the neutral molecule and the molecular ion. Spectra were obtained at two locations in the shock layer produced by a 20-cm long cylindrical blunt body inserted in the arc jet conical flow. The technique used for determining the temperatures from the spectral radiation involves obtaining ratios of integrated intensities for certain spectral regions of the measured spectrum and making a comparison with computed ratios as a function of temperature for the same spectral regions. The technique is difficult to apply due to radiation overlapping bands from other species. However, using a trial and error procedure, the vibrational and rotational temperatures at the two locations are inferred. The results indicate that the shock layer is significantly out of thermal equilibrium.
A model for simulating nonequilibrium radiation from Mars entry shock layers is presented. A new chemical kinetic rate model is developed that provides good agreement with recent EAST and X2 shock tube radiation measurements. This model includes a CO dissociation rate that is a factor of 13 larger than the rate used widely in previous models. Uncertainties in the proposed rates are assessed along with uncertainties in translational-vibrational relaxation modeling parameters. The stagnation point radiative flux uncertainty due to these flowfield modeling parameter uncertainties is computed to vary from 50 to 200% for a range of free-stream conditions, with densities ranging from 5e-5 to 5e-4 kg/m3 and velocities ranging from of 6.3 to 7.7 km/s. These conditions cover the range of anticipated peak radiative heating conditions for proposed hypersonic inflatable aerodynamic decelerators (HIADs). Modeling parameters for the radiative spectrum are compiled along with a non-Boltzmann rate model for the dominant radiating molecules, CO, CN, and C2. A method for treating non-local absorption in the non-Boltzmann model is developed, which is shown to result in up to a 50% increase in the radiative flux through absorption by the CO 4th Positive band. The sensitivity of the radiative flux to the radiation modeling parameters is presented and the uncertainty for each parameter is assessed. The stagnation point radiative flux uncertainty due to these radiation modeling parameter uncertainties is computed to vary from 18 to 167% for the considered range of free-stream conditions. The total radiative flux uncertainty is computed as the root sum square of the flowfield and radiation parametric uncertainties, which results in total uncertainties ranging from 50 to 260%. The main contributors to these significant uncertainties are the CO dissociation rate and the CO heavy-particle excitation rates. Applying the baseline flowfield and radiation models developed in this work, the radiative heating for the Mars Pathfinder probe is predicted to be nearly 20 W/cm2. In contrast to previous studies, this value is shown to be significant relative to the convective heating.
Differential approximations used to determine effects of nongray gas behavior and nonequilibrium ionization in radiating hypersonic shock layers
This paper investigates the non-Boltzmann modeling of the radiating atomic and molecular electronic states present in lunar-return shock-layers. The Master Equation is derived for a general atom or molecule while accounting for a variety of excitation and de-excitation mechanisms. A new set of electronic-impact excitation rates is compiled for N, O, and N2+, which are the main radiating species for most lunar-return shock-layers. Based on these new rates, a novel approach of curve-fitting the non-Boltzmann populations of the radiating atomic and molecular states is developed. This new approach provides a simple and accurate method for calculating the atomic and molecular non-Boltzmann populations while avoiding the matrix inversion procedure required for the detailed solution of the Master Equation. The radiative flux values predicted by the present detailed non-Boltzmann model and the approximate curve-fitting approach are shown to agree within 5% for the Fire 1634 s case.
This paper provides a description of the potential impact of turbulence on a radiating flow field with large surface blowing. This is accomplished by calculating the forebody flow field with coupled carbon-phenolic mass injection for a probe entering the Jupiter atmosphere. Both laminar and turbulent flow conditions are assumed. For the no blowing solutions, turbulence is shown to have no impact on the surface radiative heating. However, with ablation injection, turbulence significantly increases the surface radiative heating rate. This occurs because the turbulence brings the high temperature gases closer to the surface, thus thinning the cool molecular gases at the wall which are responsible for blocking much of the radiation.
An implicit finite difference scheme is developed for the fully coupled solution of the viscous radiating stagnation line equations, including strong blowing. Solutions are presented for both air injection and carbon phenolic ablation products injection into air at conditions near the peak radiative heating point in an earth entry trajectory from interplanetary return missions. A detailed radiative transport code that accounts for the important radiative exchange processes for gaseous mixtures in local thermodynamic and chemical equilibrium is utilized.
NASA's In-Space Propulsion program is supporting the development of shock radiation transport models for aerocapture missions to Mars. A comprehensive test series in the NASA Antes Electric Arc Shock Tube facility at a representative flight condition was recently completed. The facility optical instrumentation enabled spectral measurements of shocked gas radiation from the vacuum ultraviolet to the near infrared. The instrumentation captured the nonequilibrium post-shock excitation and relaxation dynamics of dispersed spectral features. A description of the shock tube facility, optical instrumentation, and examples of the test data are presented. Comparisons of measured spectra with model predictions are also made.
This paper presents a review of the analysis and measurement of radiation data obtained in the NASA Ames Research Center's Electric Arc Shock Tube (EAST) facility. The goal of these experiments was to measure the level of radiation encountered during atmospheric entry. The data obtained from these experiments is highlighted by providing the first spectrally and spatially resolved data for high speed Earth entry and measurements of the CO 4th positive band for conditions relevant to Mars entry. Comparisons of the EAST data with experimental results obtained from shock tunnels at JAXA and the University of Queensland are presented. Furthermore, the paper will detail initial analyses in to the influence and characterization of the measure non-equilibrium radiation.
Fully coupled solutions of shock layer equations for the stagnation region are obtained, comprising a set of converged benchmark flowfield solutions for silica and carbon heat shields entering modeled atmospheres of Saturn and Uranus. Model atmospheres and entry trajectories providing significant radiative heating to the entry probes so that the heating environment and effects of mass addition on the heating environment are clearly defined are emphasized. Results referable to the carbon heat shield are questioned because of the high (sublimation) wall temperature assumed and the relatively low shock layer temperature.
Spectral and total radiation studies of bodies traveling at hypersonic speed in simulated Martian and Venusian atmospheres
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